Not applicable
Not Applicable.
This invention relates to elastomeric bushings, and in particular to an improved bushing for use in heavy off-road and over-the-road vehicles. The bushings of the present invention are particularly, but not exclusively, useful in replacing rubber and polyurethane bushings in a lift axle suspension system for such heavy vehicles.
It is common to provide lift axles for certain heavy hauling vehicles such as dump trucks, refuse haulers, grain trucks, ready-mix cement transit mixers, block haulers, construction vehicles, and other special application vehicles. Such vehicles are adapted for both roadway and off-road operation. The lift axles form a supplemental suspension system for the vehicle and provide additional support for on-road use when the vehicles are heavily laden, to assist in spreading the vehicle weight and to minimize road and bridge damage. An example of such a vehicle is the dump truck shown diagrammatically in FIG. 1. Lift axles are also used on heavy over-the-road tractors and trailers to meet government weight-per-axle limitations when the truck is heavily laden.
Lift axle suspension systems selectively lower and raise the supplemental wheels of a vehicle into and out of engagement with the road or other support surface and form a part of the support system for the vehicle when the supplemental wheels are lowered. The supplemental wheel assemblies are of various constructions, but they generally include an arm mounted to each side of the vehicle frame via one or more pivot connectors. An air bag (also called an air spring or air bellows) is commonly interposed vertically between the arm and the hanger bracket for selectively lowering the axle, wheel, and tire assembly into engagement with the road surface and providing the support for it. Another mechanism such as a second air bellows or a spring is provided for lifting the axle. The lift mechanism may also include a pivot connector. The pivot connectors are generally in the form of elastomeric bushings.
Numerous commercial lift axle suspension systems are available. They include, for example, ReycoGranning Suspensions Models LT80, LT120, T100AT, T300A, LT225, and T350AX, Granning Air Suspensions Models L120, L200, L225, BL100, T200AX, and LT120, Watson and Chalin Models LWSL-1100-SR, SL-1800, WCAL-1300, and AL2200, and Spring Valley Models AB1000, AB1000CL, AB1000AL, AB2250AL, SS120, AB1300AL, AB800CL, and TR225AR. Current commercial air lift axle assemblies have a capacity ranging from about 8,000 pounds to about 25,000 pounds. The assemblies themselves are heavy, weighing on the order of nine hundred to two thousand pounds. Although details of their constructions vary, most have either a single arm and a single bushing on each side of the frame or a two-arm pariallelogram construction having four bushings on each side of the frame.
The patent literature also contains numerous examples of lift axle assemblies, such as Narahari, U.S. Pat. No. 3,960,389, Gibson, U.S. Pat. No. 4,000,913, Taylor, U.S. Pat. No. 4,293,145, Vandenberg, U.S. Pat. No. 4,300,787, VanDenberg et al., U.S. Pat. No. 4,573,704, Hermann, U.S. Pat. No. 5,018,593, VanDenberg et al., U.S. Pat. No. 5,505,481, and Hauri, U.S. Pat. No. 5,549,322.
It has been found that existing elastomeric bushings for lift axle assemblies wear out quickly and thus need frequent replacement. Rubber, non-rotating bushings have been used traditionally. They have too much xe2x80x9cwind-upxe2x80x9d (built-up tension as they rotate) and do not allow the suspension to cycle through its entire range of motion. The substitution for conventional rubber bushings of rotatable polyurethane bushings like those described in co-assigned patents to Sturmon, U.S. Pat. Nos. 4,840,395 and 5,988,614, has not been an entirely satisfactory solution. These bushings usually require more torque to rotate than the lift axle lift spring can provide to consistently raise the axle.
All of the patents mentioned herein are incorporated by reference.
In accordance with one aspect of the present invention, generally stated, a lift axle assembly is provided for a heavy vehicle for selectively lowering and raising supplemental wheels of the vehicle into and out, of engagement with a support surface, the assembly forming a part of the support system for the vehicle when the supplemental wheels are lowered, the assembly comprising at least one bushing, the bushing comprising a hard sleeve having a hardness of greater than Shore 65D, an elastomeric sleeve surrounding the hard sleeve, and a shaft rotatably mounted in the low friction sleeve. The shaft is typically in the form of a bolt, a solid pin, or a hollow tube. The low friction sleeve has a coefficient of friction lower than the elastomeric sleeve. In a preferred embodiment, the low friction sleeve is a self-supporting tube. In a particularly preferred embodiment the low friction sleeve is made of ultra-high molecular weight polyethylene.
In accordance with another aspect of the inventions, a heavy vehicle having a frame is provided with a moveable arm, and a pivot connecting the moveable arm to the frame, the pivot comprising a bushing having a first part operatively attached to the frame and a second part attached to the arm, the second part being coaxial with the first part, wherein one of the first and second parts comprises a hard polymeric sleeve and an elastomeric sleeve surrounding the hard sleeve, and the other of the first and second parts comprises a metal shaft rotatably mounted in the hard sleeve. The sleeve is preferably self-supporting and made of a hard polymer.
Typically, but not necessarily, the shaft is rigidly connected to the vehicle body and the sleeves are mounted in the rotatable arm. The arm has a substantial angular travel from the assembly""s retracted position to its lowered position. In most designs it rotates at least ten degrees but less than fifty degrees, usually on the order of fifteen to thirty degrees. It has been found that the use of the bushing of the present invention greatly reduces the effort required to rotate the lift axle from its road-engaging position to its retracted position. It has been found that as little as ten foot pounds of torque, and typically no more than fifty foot pounds of torque, is required to turn the sleeve with respect to the shaft, even when the bushing is clamped tightly or after long continuous use in the road-engaging position. The bushing also increases bushing life by reducing torque-induced stress on the elastomeric component.
Preferably, the bushing is substantially free of lubricant.
Preferably, and in accordance with one aspect of the invention, the elastomeric sleeve is made of polyurethane. As used herein, except as otherwise indicated, the term xe2x80x9cpolyurethanexe2x80x9d includes polyurethanes, polyureas, and blends thereof. The elastomer preferably has a Shore (durometer) hardness in the range of 65A to 95A. The elastomeric sleeve, for many applications, typically has a hardness of about Shore 85A to 95A. A particularly preferred material has a tensile strength of at least 4,000 psi, a tear (die C) strength of at least 525 pli, a 100% modulus of over 2,500, and an ultimate elongation of at least 100%. An ultimate elongation in the range of 100% to 300% is preferred. Other elastomeric materials may also be used, but are not preferred.
The hard sleeve is made of a material having a much higher Shore hardness, in the range of about 65D or higher. It also has a lower coefficient of friction than the elastomeric sleeve. The preferred material is a polyolefin, such as a polyethylene or a polypropylene; particularly preferred is an ultra-high molecular weight (UHMW) polyethylene having a weight average molecular weight of 3.1 million or greater. Although more expensive, a self-lubricating acetal or a polytetrafluoroethylene sleeve is believed also to be operable in the invention. Although less preferred, other polymers such as hard polyurethane (65D or greater), nylon and phenolic (Garolite) resins may also be workable. For some extreme applications, the hard sleeve may be a metal, such as bronze, although this modification is generally not preferred.
The hard polymeric sleeve is preferably fixed to the elastomeric sleeve. In the preferred embodiments, the elastomeric sleeve is heated sufficiently to adhere to the hard plastic sleeve. The inner sleeve is preferably about 0.1xe2x80x3 to 0.5xe2x80x3 (0.25-1.27 cm) thick. The outer elastomeric sleeve may be 0.2xe2x80x3 to 2.5xe2x80x3 (0.5-6.35 cm) thick.
The bushing may optionally include an outer metal shell surrounding the elastomeric sleeve. The elastomeric sleeve may be bonded to the shell, but need not be.
Preferably, the arm is part of a lift-axle assembly carrying a wheel. The lift axle assembly may be either a pusher or a tag configuration. In this environment, the bushing of the present invention has been found to provide remarkable decreases in the forces required to rotate the bushing or shaft and to provide exceptionally long life, while providing vibration dampening, self-alignment, and axial stress relief at least comparable to that of known bushings.
The bushing of the present invention may also be used in other heavy vehicle applications, such as the assemblies used for raising and lowering cement mixer mixing thimbles, and heavy vehicle steering axles. The bushing of the present invention, however, is not believed to be suitable for all uses in heavy vehicles. It will not tolerate large side loads without support, and its direct loading capacity is limited. Therefore, although it is highly suited to uses in which maximum loads on the bushing are on the order of ten thousand pounds, it is not believed to be highly suitable for use in such vehicles for torque rod bushings or center or end bushings, because they may experience loads up to eighty thousand pounds in off-road use.
Polyurethane has been found to be a particularly advantageous elastomer. The hard sleeve can be formed by cutting sections from a standard rigid polymer tube and forcing the tube into a precast polyurethane sleeve. Preferably, the hard sleeve is a rigid tube positioned in a mold, and the elastomeric is poured and cured around it. The need for machining either the elastomeric sleeve or the hard sleeve is eliminated, although the hard sleeve may be machined if desired.
Other aspects of the invention will be apparent to those skilled in the art in light of the following description of the preferred embodiments.